It has been known for some time that produce stored under refrigeration maintains its freshness for a greater time than that stored at ambient temperatures. For example, the refrigerated shipment of produce from California to the East Coast has been done since the 1920's. It is not as widely known but is familiar to those in the business that controlling the atmosphere where storage is being undertaken extends the lifetime of the produce.
An important example is the storage of apples and other fruit in Washington State from one fall harvest to another. To provide proper storage the ripening process must be retarded. During the ripening process the starches in the fruit are converted to sugars through natural respiration processes. Ethylene gas is released and accelerates the process. With refrigeration alone the ripening process continues at a slower rate unless the temperature is reduced to a point where the fruit freezes resulting in considerable deterioration. To remedy this problem it has been found desirable to place the fruit into a dormant stage by placing it in a reduced oxygen atmosphere and a lowered temperature. Reduction of temperature is commonly done with large refrigeration units both in the prior art and the invention.
There are several methods in present use for production of the proper atmosphere. The simplest method is flooding the area with nitrogen separated by liquidation of air. The nitrogen is supplied either in compressed or liquid form. While this method has the advantage of simplicity it is quite expensive to maintain the proper atmosphere and thus not met with widespread adoption. The method in most common use in the Washington State apple business is to burn out the oxygen and remove combustion products. To accomplish this end propane is injected into an incoming air stream and ignited, generally by a catalytic burner. The resulting products of combustion are nitrogen, water vapor and carbon dioxide. The carbon dioxide is harmful to the produce and must be removed. The removal of carbon dioxide is commonly done by devices called scrubbers. One type of scrubber uses sodium or potassium hydroxide solutions to absorb the carbon dioxide forming sodium or potassium carbonate as a waste product. Similarly, calcium hydroxide (lime) has been used to remove the carbon dioxide. These chemical scrubbers are expensive to maintain and require constant addition of reagents with removal and storage of wastes. Problems with scrubbers have led to large scale introduction of molecular seives as removal means. A molecular seive will absorb a gas such as carbon dioxide at one temperature then release the carbon dioxide at an elevated temperature. In operation the seive is cycled between absorption and regeneration states requiring heating and cooling at each step.
The primary problems with the use of catalytic burners and molecular seives are related to cost of operation. These are largely related to the rising cost of energy. Since the atmosphere is reused the input of atmosphere to the catalytic burner is on the order of 0.degree. C. This atmosphere must be heated to the ignition point of the propane mixture even in a catalytic burner. The cost of the energy to do so is not unsubstantial. In addition the molecular seive must be heated on its regeneration cycle further increasing energy consumption. There must be an excess of oxygen to ensure formation of carbon dioxide in the burner rather than carbon monoxide. This requirement puts an upper limit on the possible oxygen reduction in this type of system. Finally, this type of a system does not remove ethylene gas formed from the fruit which accelerates ripening.
Accordingly, there is a demand for an atmosphere control system which (1) operates at a lower temperature than existing systems, (2) controls oxygen content to less than 2.5% to reduce the conversion of starch to sugar, (3) removes almost all carbon dioxide from the atmosphere and (4) removes ethylene gas from the atmosphere.